In typical applications of liquid or powdered coatings, problems encountered include the use of volatile organic contaminants to suspend the coating solids, the overspray and the necessity of masking to assure accurate coverage, and the use of large amount of thermal energy in curing. These problems are costly in terms of pollution abatement, high labor content, and high energy consumption. The abatement of volatile organic contaminants is strictly controlled by the EPA in terms of pollution control and possible exposure to toxins. Strict adherence to EPA guidelines by the manufacturers is both costly and time consuming.
The accurate control of coating coverage is frequently difficult and requires extensive masking even in more advanced coating techniques such as the electrostatic coating. The application and removal of masking is time consuming and labor intensive. Another poor use of time and energy is the necessity of curing a coating in a hot air oven.
In an effort to overcome the drawbacks encountered in conventional coating techniques, coating methods by using laser have been developed in recent years. These techniques normally utilize laser for curing an already applied coating on a substrate. The laser energy is used very inefficiently since the exact location of the laser beam and its energy distribution are not taken into consideration. For instances in curing a coating having a high melting point, the curing point is set at or near the focus point of the laser in order to achieve a high power density. In curing a coating having lower power requirements, the laser beam is defocused which makes the thermal transfer less efficient. The only available process control is to increase the power level in order to overcome any curing problems.
The presently available laser curing techniques do not utilize gas flows to carry a coaxial flow from a lens to a substrate. Some laser techniques require the operation to be conducted in sealed atmospheric chambers which is not a practical approach for high volume productions. For instance, one existing method of laser curing of powder coating applies a thin layer of powders on a substrate first and then irradiates a laser beam over the powders. Since the laser beam causes the powders to draw together, several successive layers of powders must be applied and the laser beam must be used to pass over the coating several times. This makes a continuous process or a process capable of accurate thickness control very difficult. In coating high temperature powders such as those containing metal particles or refractory materials, a laser beam first forms a molten pool of the substrate and then powder is sprayed directly onto the molten area. As a result, the substrate material is frequently damaged by the intense heat.
Other laser coating techniques involve the use of powder coatings that contain special laser light absorbers mixed into the powder in order to increase the absorption of laser energy. This is only necessary because of the inefficient use of laser energy in conventional laser coating techniques, where most of the energy is wasted. Still other conventional laser coating techniques involve the use of high energy densities and relatively low coating speeds. Powders are applied either electrostatically or simply sprayed on with poor repeatability. A slow laser scanning speed is necessary in order to ensure that all the powder is melted in one pass. The powder can also be indirectly heated from below by the heat transferred from the substrate. It is therefore not possible, using a conventional laser curing technique, to coat plastic substrates except at slow speeds and at small powder thicknesses. Since the laser is applied to the powders inefficiently, an excessive amount of heat must be generated in order to carry out a conventional laser coating technique.
It is therefore an object of the present invention to provide a method and apparatus for using a laser in coatings that do not have the shortcomings and the drawbacks of the prior art laser coating methods.
It is another object of the present invention to provide a method and apparatus for using a laser in coatings that do not require the processing step of first applying a layer of powder on a substrate prior to the application of laser energy.
It is a further object of the present invention to provide a method and apparatus for using a laser in coatings in which a coating powder is first energized with laser energy before it is applied and fused to a substrate.
It is yet another object of the present invention to provide a method and apparatus for using a laser in coatings by forcibly inserting a cloud of coating particles into a laser beam attenuated by defocusing.
It is still another object of the present invention to provide a method and apparatus for using a laser in coatings by first providing an inert gas to form a cloud of coating particles suspended in the gas in such a way that the inert gas serves as a shield against combustion.
It is another further object of the present invention to provide a method and apparatus for using a laser in coatings by first providing an inert gas to form a cloud of coating particles suspended in the gas and energized by a laser beam such that the coating particles can be directed downwardly toward a substrate in addition to the sideway spreading of the particles.
It is still another further object of the present invention to provide a method and apparatus for using a laser in coatings wherein coating particles suspended in an inert gas begin to melt and flow as soon as the particles are energized by a laser beam.
It is yet another further object of the present invention to provide a method and apparatus for using a laser in coatings wherein coating particles suspended in an inert gas begin to melt and flow as soon as they are energized by a laser beam such that the particles adhere to a substrate instantly.